1. Field of the Invention
This invention relates to polymeric materials. The polymeric materials are particularly suitable for biomedical applications, such as a component of an intraocular lens.
2. State of the Art
Polymers have been used in biomedical applications for a long time. Early in vivo studies on polymeric implants revealed that the polymers are susceptible to degradation in physiological environment and lose integrity over time. A close scrutiny of the structure and bio-properties relationship led to Pinchuk's discovery of the superior biostability of polyisobutylene-based materials. The biomedical application of polyisobutylene-based materials is disclosed in U.S. Pat. Nos. 5,741,331; 6,102,939; 6,197,240; 6,545,097; and 6,855,770, all of which are herein incorporated by reference in their entirety. The first commercial application of such materials is the use of SIBS in the TAXUS® Stent of Boston Scientific Corporation, which is regarded as the most successful launch of a biomedical device in history.
SIBS is a thermoforming triblock copolymer consisting of polyisobutylene (PIB) as the rubbery center block and polystyrene (PS) as the hard side blocks. Due to the immiscibility of PIB and PS, the SIBS material has microphase-separated morphology in which PS phase forms physical crosslinks in the matrix of rubbery PIB phase. Due to the thermoplastic nature of the crosslinking, SIBS material creeps and can lose its dimension. SIBS doesn't withstand the high temperature of autoclave sterilization due to limitation by the glass transition temperature of PS. As a result, SIBS sterilization is difficult, because gamma-sterilization breaks down SIBS and ethylene oxide sterilization is cumbersome.
PIB is commonly crosslinked through vulcanization. First, isobutylene is copolymerized with a small fraction (1-5%) conjugated dienes such as butadiene, so that there are carbon-carbon double bonds in the backbone providing sites for vulcanization; second, the isobutylene/butadiene copolymer is heated with sulfur and crosslinked by the sulfur. To accelerate the vulcanization process, either the polymer is activated as in the case of halo-butyl rubber, or accelerators are added such as resins, zinc oxide, xanthates and quinoid systems. An extremely fast vulcanization process involves mixing butyl rubber solution with sulfur monochloride at room temperature (Erman et al, Macromolecules, Vol. 33, 2000, 4822-4827). The chemicals used for vulcanization of butyl rubber are toxic to human body. Extraction by solvent is necessary for removal of toxic residuals, but complete extraction is difficult and time-consuming.
PIB can be crosslinked utilizing silicone chemistry. Kaneka developed telechelic functional PIB (trade name: Epion) with silyl or allyl end groups, which can be crosslinked by moisture or adding silanes. Faust et al disclosed a virtually telechelic silyl PIB, which undergoes room temperature crosslinking as described in U.S. Pat. No. 6,268,451.
Benzocyclobutene derivates and 1-hexene were analyzed in Fishback et al., “A New Non-Toxic, Curing Agent for Synthetic Polyolefins,” Bio-Medical Materials and Engineering, Vol. 2, pp. 83-87 (1992), herein incorporated in reference in its entirety. Fishback prepared polymers containing 1-hexene, allyl-benzocyclobutene, and a diene, either 7-methyl-1,6-octadiene or 5-methyl-1,4-hexadiene, using free-radical polymerization techniques. While the polymer showed improved properties, it requires carbon black as a filler, and is polymerizable only through free-radical chemistry techniques that necessitate the use of free radical initiators. Additionally, there is a need to extract the non-crosslinked polymers to rid the system of the initiators as the initiators, if not removed, would leave the polymer with an undesirable purple color.